1. Field of the Invention
This invention relates generally to an automatic transmission, and more particularly to its control during a transition from Park to a forward drive range or a reverse drive range.
2. Description of the Prior Art
When rolling movement a vehicle is produced by placing a range selector of an automatic transmission in the Park range, the vehicle wheels are held against rotation by securing them through a drive connection to the transmission case. It is conventional practice to engage automatically a friction control element of the transmission that must be engaged in both first gear and reverse gear, when Park range is engaged.
When a transmission in this state is shifted out of Park on a grade, a large objectionable boom can occur when the stored energy of the loaded drive line is suddenly released. This abrupt release of energy can cause the driveline to oscillate for a brief but undesired period.
A need exists in the industry for a reliable technique that prevents unintended rolling of a vehicle using a park brake, and avoids a harsh, abrupt release of energy stored in the vehicle driveline when the park brake is released.